This invention relates to assemblies of semiconductor devices and particularly to assemblies accurately positioning disc mount semiconductor devices in relation to heat sink/bus bar elements.
Disc mount semiconductor devices are those in which a semiconductor device is encapsulated in a package having opposing circular faces to which pressure contact is made, without bonding, for electrical continuity to regions of the enclosed device as well as for heat transfer. Such packaged devices are well known and widely used and may include either diodes, transistors, or thyristors, in accordance with known practice. The term "hockey puck" is often used in referring to devices encapsulated in this manner because of similarities of appearance. Such devices are also sometimes referred to as compression bonded encapsulated (CBE) devices.
The nature of disc mount devices is such that it is necessary to apply a predetermined force, typically in a range from about 1000 to about 2000 pounds, that is accurately positioned, typically specified as within 1/32 of an inch, at the central axis of the device in order to ensure good thermal and electrical contact. In use, one or more of the semiconductor devices, frequently several, are stacked with intermediate sheet metal elements that serve as heat sinks and electrical conductors. In assembly, it is necessary for the assembler to ensure the proper axial alignment of the devices so that the stack will have the required force applied along the central axis of the devices. A misplacement of a device by more than about 1/32 of an inch may be fatal to its reliable operation because of the wrong amount of pressure on the contact faces, sometimes referred to as pole faces.
The art has addressed the problem of accurate location of disc mount devices by adopting a standard device and heat sink configuration so they are intended to mate and provide accurate location. In accordance with this technique the semiconductor device manufacturer provides in each major face of the device a hole or indentation that is accurately centrally located and which is to receive a pin or bump that is integrally formed with the heat sink/conductor element. In concept, this requires the assembler merely to position the device with the central aperture located on the pin in the sheet metal element. In practice, this takes a considerable amount of time on the part of the assembler to ensure accurate location and may result in misalignment and device failure. For example, the device may be located so that the pin is not aligned with the aperture because it is so small the assembler may not feel the misalignment. Upon clamping the assembly to provide the required amount of pressure, the pin may penetrate the relatively soft metal, such as copper alloy, pole face of the device, with serious detriment to device performance. The assembly requires a high degree of sensitive feeling on the part of the assembler because of the inability to view the location of the pin from the sheet metal heat sink within the pole face aperture.
Disc mounted semiconductor devices and their mounting by the pin-in-hole technique, as well as clamping means therefore, are generally described in Westinghouse Application Data, Mounting Power Semiconductors, January 1978, especially pp. G47-G49, which is herein incorporated by reference.
It is understood that others have at least attempted to provide some form of peripheral and visible locating means for disc mounted devices on heat sinks, such as a centering ring in which the device is located with the centering ring attached to the heat sink. Unfortunately, this requires precisely dimensioned centering rings and is inherently undesirable because of requiring an additional part member.
The present invention achieves the required precise axial location of disc mounted devices, whether used singly or in a multiple stack, in relation to their adjacent heat sink/conductor elements without requiring any additional part members and without requiring any great care on the part of the assembler.
In accordance with the present invention, each of the metal sheets for making electrical and thermal contact with respective contact faces of a device is provided with mechanically deformed elements of the sheet metal material that engage the edge surface of the device and hold it in a position so that a stack of such devices is accurately located on the axis. The deformed elements are preferably a plurality of holding fingers stamped from the material of the sheet and deformed to provide edges that grip the adjacent device. This type of assembly requires merely that there be accuracy in the die used to stamp the fingers from the sheet metal. The assembler has merely to force, with a minor interference fit, the device into the array of peripheral fingers, which may number three or more, in order to ensure accurate location.
A single sheet metal element that serves to contact two adjacent devices with electrically common faces is formed by having stamped fingers that protrude from each of its major surfaces that are located on the same circle so that the two adjacent devices are certainly located in the proper position.
The assembly not only relieves the assembler of any high skill in assembly but it also permits by direct viewing the assurance that the device is accurately positioned. This makes it possible not only to position the device easily but to make sure that both before and after the application of the clamping pressure accurate positioning is maintained.